Low birth weight has become a target of U.S. and international health policy, therefore it is important to study the effects of birth weight on children's health and development. The specific aims of our study are - (1) To estimate the impact of birth weight on child outcomes using data on twins (2) To examine whether the impact of birth weight varies across various child outcomes (3) To test whether specific post-natal parental investments compensate for or reinforce the impact of birth weight on child outcomes. The above research aims will be addressed using data from the first two waves of the Early Childhood Longitudinal Study - Birth Cohort (ECLS-B), which follows a nationally representative sample of births in the U.S. in the year 2001 from infancy until first grade, and provides information on their cognitive, health and developmental outcomes during this period. The oversampling of twins as well as low- and moderately-low birth weight babies in this dataset allows us to estimate twins fixed effect (TFE) models. Estimates based on the sample of singleton births will be compared to TFE estimates from samples of fraternal and identical twins. TFE estimates using fraternal twins'controls for all maternal factors and TFE estimates using identical twins additionally controls for genetic factors. Rich data on early childhood cognitive, developmental and health outcomes will allow us to examine whether birth weight effects vary across outcomes using the same sample. Finally, rich data on a variety of post-natal investments allow us to examine whether parents make differential investments in their higher birth weight child compared to their lower birth weight child. This work will provide new knowledge concerning the relationships between birth weight and children's cognitive, developmental and health outcomes. By estimating birth weight effects using twins data, this study will help distinguish the policy-relevant effects of increasing birth weight from possible genetic and family background influences on fetal development.